The present invention relates to the art of instrumentation of a petroleum well during drilling and, more in particular, to a device to monitor the level of drilling mud in a hold being drilled.
Drilling mud in petroleum wells being drilled provides a substantial column of high gravity fluid that resists the pressure of downhole formation fluid and prevents the latter fluid from escaping the well and causing a blowout. Drilling mud also carries away chips generated in the drilling zone by a drilling bit and lubricates the bit. Drilling mud passes down through a drill string and exits at the base of the string into the formation being drilled. The mud returns to the surface, typically between a casing and the drill string. On the surface the mud is cleansed of the cuttings.
If the level of drilling mud gets too low the resistance to blowout decreases and the risk of blowout increases. When tripping pipe of a drill string, it is desirable to let the mud level drop a little so that pipe sections come from the hole comparatively dry and mud is not wasted. However, if the level drops too low, blowout can occur.
U.S. Pat. No. 2,832,566 to Bielstein discloses a means for monitoring the level of drilling mud in a hole. This patent uses a column of drilling mud in an annulus between a drill string and a casing to oppose a column of air in communication with the column of drilling mud. The level of the drilling mud varies in service. The column of drilling mud in opposition to the air varies in accordance with these variations. The patentee teaches the use of air pressure balancing the column of drilling mud as being an indication of the level of drilling mud. When the pressure drops to atmospheric indicating no drilling mud opposition, a pressure switch closes a circuit to alarm devices.
Bielstein's sytem does not provide a record of drilling mud level as a function of time. Further, there is no indication of air flow rate and so there is no continuous indication of drilling mud level.